Video display protection circuit

ABSTRACT

An apparatus and a method for protecting a video display device, having an anode and a cathode, against excessive beam current conditions. In particular, the present invention protects the video display device against zero-biasing of the device, wherein the cathode driver circuit is supplied by a plurality of power supplies and the loss of anyone of the power supplies causes zero-biasing of the cathode. The invention provides, in a video display device ( 100 ) having an anode ( 112 ) and a cathode ( 102 ) for generating and controlling beam currents in the display device, a protection circuit comprising: a source of video signals ( 36 ); a driver circuit ( 40 ) coupled to the source of video signals and the cathode ( 102 ), the driver circuit coupled to a driver circuit power supply (+12V,+215V), the driver circuit causing a cathode voltage to be generated on the cathode in response to the video signals and the driver circuit power supply; a high voltage power supply (X 1 ) coupled to the anode ( 112 ) for providing high voltage accelerating potential in the display device; and a shutdown circuit ( 44 ) coupled to the driver circuit power supply and the high voltage power supply, the shutdown circuit disabling the high voltage power supply if the driver circuit power supply voltage decreases below a threshold level.

This application claims the benefit under 35 U.S.C. §365 ofInternational Application PCT/US99/22660, filed Sep. 29, 1999, which waspublished in accordance with PCT Article 21(2) on Apr. 6, 2000 inEnglish, and which claims the benefit of U.S. Provisional ApplicationSerial No. 60/102,213, filed Sep. 29, 1998.

The present invention relates to a video display protection apparatusand method, and more particularly, to a video display protection circuitfor protecting a cathode ray tube from a high beam current condition.

In a video display device comprising a cathode ray tube (CRT), beamcurrent is generated in response to a video signal and accelerated by anaccelerating potential to cause the beam to strike a phosphor coatedscreen. In such a display device, several different voltages arerequired to generate the beam current, focus the beam and accelerate thebeam toward the screen. These voltages include a filament excitationvoltage, a modulated cathode voltage, a grid 1 voltage, a screen grid 2voltage, focus grid voltages, and a final anode voltage.

The filament voltage (nominally 6.3V) excites the filaments to generatefree electrons to supply the electron beam in the CRT. A cathode voltageis generated on each of the R, G, and B cathodes and controls the amountof beam current developed in the CRT. The focus grid voltages generatean electrical field that functions in a lens-like manner to focus thebeam in the midst of its trajectory. The final anode voltage, nominally30-32 KV, accelerates the beam and electron beam currents will not flowfrom the cathode to the anode in the absence of the final anode voltage.

The beam current is a function of the cathode voltage, grid 1 voltageand a cutoff voltage. More specifically, the beam current corresponds toVc-(Vk-Vg1), wherein Vc is the cutoff voltage, Vk is the cathode voltageand Vg1 is the grid 1 voltage. The cutoff voltage is set based on theCRT characteristics and in view of the screen grid voltage. Since thebeam current is generally controlled by modulating the cathode, insteadof modulating the grid 1 voltage, and beam current is an exponentialfunction of the control voltage, this discussion uses notation that isinverse in sign from physical theory. Thus, for purposes of the presentdiscussion, an increase in screen grid voltage results in increased Vc.

If the grid 1 voltage is approximately zero, and Vc is set to beslightly less than the kinescope driver supply of about +215Vdc, thevariation in the cathode voltage Vk controls the amount of beam currentthat flows in the CRT. For cathode voltage Vk close to Vc, beam currentis negligible, while for small Vk, substantial beam current may result.

The cathode voltage is typically generated in response to a video signalby a kinescope driver circuit that includes a multi-stage amplificationcircuit. The multi-stage amplification circuit is usually coupled to atleast a low voltage power supply and a high voltage power supply. Themulti-stage amplification circuit may be embodied in the form of anintegrated circuit, for example TDA6120Q, manufactured by PhilipsSemiconductors of Sunnyvale, Calif. It is advantageous to use ICs suchas TDA6120Q in high performance video monitors because of theirrelatively high bandwidth and ability to handle relatively highvoltages.

However, in some cases, particularly when the kinescope driver circuitis embodied in the form of an IC, such as the TDA6120Q, the loss ofeither the low voltage or high voltage power supply to a kinescopedriver circuit may result in the loss of cathode voltage. The loss ofcathode voltage results in a “zero-biasing” of the CRT, which results inan extremely high beam current. The high cathode current is especiallyproblematic in video display monitors that already operate at relativelyhigh beam currents. High beam currents are desirable to increase thebrightness of the display and thereby improve the subjective quality ofthe picture. However, faults in the kinescope driver circuitry powersupplies can easily lead to high beam currents that can permanentlydamage the phosphors on the display screen. In this connection, faultsin the kinescope driver circuitry power supplies may occur due to avariety of different reasons including, but not limited to, break in thewire during manufacturing and/or shipment, and oxidation of the variouselectrical components.

Currently, safety circuits shutdown the anode voltage supply if theanode voltage levels exceed predetermined levels, in order to preventexcessive x-radiation levels. However, such safety circuits respond tothe voltage level of the anode voltage supply, but do not respond to aCRT zero-bias condition that results from faults in the power suppliesto the kinescope driver circuit.

Therefore, it is desirable to provide an apparatus and a method forprotecting a video display device against a zero-bias condition due tothe loss of any one of a plurality of power supplies, which conditionresults in excessive beam current in the video display device.

In particular, it is desirable to provide an apparatus and a method forprotecting a video display monitor against a zero-bias condition due tothe loss of any one of a plurality of power supplies associated with anyone of a plurality of kinescope driver circuits.

It is also desirable to provide an apparatus and a method for protectinga video display monitor against a zero-bias condition due to the loss ofpower to a kinescope driver circuit, wherein the kinescope drivercircuit is embodied in the form of an integrated circuit.

It is also desirable to provide an apparatus and a method for protectinga video display monitor against the loss of a cathode voltage, whichapparatus and method is self-latching and does not reset until the faultis removed.

It is also desirable to provide a video display apparatus and methodwhich disables a high voltage power supply when a fault is detected inany one of a plurality of driver circuit power supplies.

“In an exemplary embodiment, the present invention comprises aprotection circuit that shuts down the anode power supply in response tothe loss of a power supply to a kinescope driver circuit. In particular,the present invention is a protection circuit comprising: a source ofvideo signals; a driver circuit coupled to the source of video signalsand the cathode, the driver circuit coupled to a driver circuit powersupply, the driver circuit causing a cathode voltage to be generated onthe cathode in response to the video signals and the driver circuitpower supply; a high voltage power supply coupled to the anode forproviding high voltage accelerating potential in the display device; anda shutdown circuit coupled to the driver circuit power supply and thehigh voltage power supply, the shutdown circuit disabling the highvoltage power supply if an output of the driver circuit power supplydecreases below a threshold level.”

The invention will be described with reference to the accompanyingdrawings, wherein:

FIG. 1 is a block diagram of a video display protection circuit inaccordance with the present invention;

FIG. 2 is a schematic diagram including the summing node for the lowvoltage and high voltage cathode supplies in accordance with the presentinvention; and

FIG. 3 is a schematic diagram of a high voltage power supply andshutdown circuit in accordance with the present invention.

FIG. 1 illustrates a block diagram of video display apparatus whichincludes a protection circuit in accordance with the present invention.The apparatus comprises power supply board 22 coupled to Input/Outputboard 24 and kinescope driver board 20 via connector cables 31, 32, 35and 45. Power supply board 22 includes circuitry associated with +12Vsupply 25, +215V supply 27 and High Voltage supply 26. +12V supply 25and +215V supply 27 are used to develop cathode voltage Vk as describedfurther below. High voltage supply 26 is coupled to the anode andprovides the accelerating potential.

CRT 100 provides a video display in response to drive signals fromkinescope driver board 20 and high voltage from power supply board 22.Each cathode of CRT 100 is driven by a respective output from kinescopedriver board 20, here represented by connector 34, although a connectormay be necessary for each cathode. High voltage supply 26 is coupled toCRT 100 via connector cable 33.

I/O board 24 generally includes coupling devices that allow the user toconnect various signal sources to the CRT. In the present embodiment,+12V supply 25 is connected to kinescope driver board 20 via I/O board24 as a matter of convenience. However, it is clear that +12V powersupply may be coupled to kinescope driver board 20 in any manner asdesired.

Kinescope driver board 20 receives the video signals, and the low andhigh voltage power supplies, and develops a cathode voltage for eachcathode associated with the video signal. +12V supply 25 is coupled tokinescope driver board 20 via connector 31, I/O board 24 and connector32. +215V supply 27 is coupled to kinescope driver board 20 viaconnector 35. Within kinescope driver board 20, the various powersupplies are coupled to each other at sensing node 30, via respectiveresistors R1 and R2. The voltage at sensing node 30 is coupled to powersupply board 22 via diode D1 and resistor R4. The sensing nodesassociated with other cathode driver circuits are connected to node 43via diodes D2 and D3. As described further below, the present protectioncircuit is arranged so that if a fault is detected in either powersupply 25 or power supply 27, via summing node 30 and connector 45, highvoltage supply 26 is shutdown by shutdown circuit 44. The fault mayresult from the failure of the respective power supplies or variousconnectors becoming disconnected.

FIG. 2 is a schematic diagram that shows portions of kinescope driverboard 20, protection delay circuit 47 and shutdown circuit 44. Kinescopedriver board 20 includes cathode driver ICs 40, 41 and 42 for each ofthe red, green and blue video signals. The circuitry for each of thecomponent colors is identical and only the components associated withthe red signal will be described. A suitable driver IC includes, but isnot limited to, TDA6120Q manufactured by Philips Semiconductors. DriverIC 40 is coupled to source of signals 36 for receiving the video signalused to control the output to cathode 102. Driver IC 40 includes amulti-stage amplifier disposed therein, and is coupled to a +12V supplyat pin 6 and a +215V supply at pin 10. The output of driver IC 40 isprovided at pin 1 2 and controls the cathode voltage of red cathode 102.As discussed above, the cathode voltage controls the beam current in CRT100.

The +12V power supply is also connected to sensing node 30 via resistorsR5 and R2. The +215V power supply is connected to sensing node 30 viaresistors R6 and R1. Sensing node 30 is coupled to combined sensing node43 via diode D1. The respective sensing nodes for ICs 41 and 42 arecoupled to combined sensing node 43 via respective diodes D2 and D3. Thevoltage at sensing node 43 is indicative of whether a fault exists onany of the power supplies to ICs 40, 41 and 42. Sensing node 43 iscoupled to the base of Q2 of shutdown circuit 44. Diodes D1, D2 or D3will conduct if either the +12V supply or +215V supply to any of the ICs40, 41 and 42 is below a threshold value. If any one of the diodes D1,D2 or D3 conduct, the voltage at combined sensing node 43 goes low,causing shutdown circuit 44 to disable the high voltage drive circuit asdescribed further below. In this manner, a fault in either one of thelow or high voltage supplies to any of the ICs 40, 41, and 42 causes thehigh voltage supply to the anode to be shutdown. Delay circuit 47prevents shutdown of the anode high voltage supply during startupconditions. Since the power supplies to ICs 40, 41 and 42 may not reachtheir normal levels until a certain finite amount of time during systemstartup, Q1 maintains the necessary voltage on sensing node 43 duringstartup to prevent activation of shutdown circuit 44.

Also, the resistors in series with the +12V and +215V supplies arefusable and will open if the ICs fail shorted. Such a failure wouldcause zero biasing of the CRT and excessive beam current. The excessivebeam current is prevented by shutting down the high voltage anode inresponse to the output of shutdown circuit 44.

37 FIG. 3 illustrates the various elements associated with shutdowncircuit 44 and the anode high voltage power supply. Sensing mode 43 isconnected to the base of transistor Q2 of shutdown circuit 44 viaconnection 45 and node 48. Shutdown circuit 44 includes a self-biasinglatch comprised of transistors Q3 and Q4. During normal operatingconditions when the +12V and +215V supplies provide the necessaryvoltages to driver ICs 40,41 and 42, transistors Q2, Q3 and Q4 are inthe OFF state. When a fault is detected at any one of the sensing nodes,the voltage at combined sensing node 43 drops below a predeterminedthreshold value, thereby reducing the voltage at node 48 and placing Q2in the ON state. When Q2 turns ON, current is supplied to the base ofQ4, placing Q4 in the ON state. When Q4 turns ON, current is supplied tothe base of Q3, placing Q3 in the ON state. This action latches Q3 andQ4 into the ON state.

During normal operating conditions, high voltage transformer X1 provideshigh voltage to anode 112. The operation of high voltage transformer X1is controlled by phase lock loop 46, a push-pull driver comprisingtransistors Q5 and Q6, and MOSFET Q7. Transistors Q5 and Q6 are drivenby phase lock loop 46 to generate a 10V square wave of approximately50/50 duty cycle. The square wave drives the high voltage generatorMOSFET Q7, which energizes high voltage transformer X1. When PLL 46drives Q6 to the ON state, the Vcesat of Q6 drives the gate of Q7 belowthe threshold voltage required for Q7 to conduct, thereby keeping Q7 inthe OFF state. When PLL 46 drives the base of Q6 high, R28 provides basecurrent to Q5, supplying MOSFET Q7 with 12V-Vcesat, saturating MOSFET Q7and energizing the primary of high voltage transformer X1 with current.

When PLL 46 again provides drive to Q6 and removes drive current fromQ5, the voltage at the gate of MOSFET Q7 falls below the thresholdvoltage thereby placing MOSFET Q7 in the OFF state. When MOSFET Q7 turnsOFF, the resonant circuit formed by the primary of high voltagetransformer X1 and the retrace capacitor C10 present a resonant retracepulse of approximately 1000V across the primary of high voltagetransformer X1. During retrace, the energy stored in the core of thehigh voltage transformer X1 is transferred to the high voltage winding,or “tertiary”, in the form of an impressed voltage of approximately theprimary pulse times the turns ratio. This results in a tertiary chargingcurrent, which provides charge to anode 112 of CRT 100 as long as thetertiary diodes are forward biased.

The primary supply voltage, B+, is adjusted to maintain a constant anodepotential for a variety of frequency and load conditions. For multiplefrequency operation, switching of the high voltage generator issynchronized by PLL 46, synchronizing the first loop, the high voltagegenerator, with the horizontal deflection, and sampling the high voltagetransformer X1 retrace pulse for establishment of the second loop errorsignal.

In the event of a fault which results in a loss of kinescope driverboard power supplies, which may cause excessive beam current, the sensedvoltage at node 48 causes Q2 to be placed in the ON state. When Q2 goesto the ON state, the self-biasing latch comprised of Q3 and Q4 goes tothe ON state thereby drawing current away from the base of Q5. Thisresults in the gate voltage of MOSFET Q7 becoming insufficient to exceedthe threshold voltage necessary to turn ON MOSFET Q7. As a result, noenergy is stored in the core of high voltage transformer X1. If there isany beam current, the anode is quickly discharged, in the absence of atertiary charging current. As the anode is discharged, there is no anodeaccelerating potential to allow damaging beam current. The self-biasinglatch comprised of Q3 and Q4 removes the high voltage power supply tothe anode until all power supplies are removed, and then restored,saving the CRT from damaging conditions. The anode is quickly dischargeddue to the use of a resistive element in the negative feedback of theanode voltage to the high voltage power supply. This results in a veryfast time constant.

The present protection circuit may be used to shutdown the anode voltagein response to a high voltage level. It is necessary to shutdown theanode voltage if the high voltage exceeds a predetermined thresholdlevel in order to prevent excessive x-radiation from being generated.Here, the output from an X-ray protection sensing circuitry is coupledto the base of Q4 via the emitter of Q8. As such, if the output of theX-ray protection sensing circuitry indicates an excessively high voltagecondition, Q8 turns ON, thereby latching Q3 and Q4 into the ON state andactivating shutdown circuit 44 as described above.

It will be apparent to those skilled in the art that although thepresent invention has been described in terms of an exemplaryembodiment, modifications and changes may be made to the disclosedembodiment without departing from the essence of the invention.Therefore, it is to be understood that the present invention is intendedto cover all modifications as would fall within the true scope andspirit of the present invention.

1. In a video display device having an anode and a cathode forgenerating and controlling beam currents in the display device, aprotection circuit comprising: a source of video signals; a drivercircuit coupled to the source of video signals and the cathode, thedriver circuit coupled to a driver circuit power supply, the drivercircuit causing a cathode voltage to be generated on the cathode inresponse to the video signals and the driver circuit power supply; ahigh voltage power supply coupled to the anode for providing highvoltage accelerating potential in the display device; and a shutdowncircuit coupled to the driver circuit power supply and the high voltagesupply, the shutdown circuit disabling the high voltage power supply ifthe driver circuit power supply voltage decreases below a thresholdlevel.
 2. The protection circuit according to claim 1, wherein thedriver circuit is disposed on an integrated circuit and the drivercircuit power supply provides at least +215V.
 3. The protection circuitaccording to claim 1, wherein the shutdown circuit comprises aself-biasing latch circuit that latches to a first state when the outputfrom the driver circuit power supply voltage decreases below thethreshold level.
 4. The protection circuit according to claim 1, whereinthe driver circuit power supply comprises first and second drivercircuit power supplies, the shutdown circuit disabling the high voltagepower supply if either one of the first or second power supply voltagesdecreases below respective first and second threshold levels.
 5. Theprotection circuit according to claim 4, further comprising a combinedsensing node that provides a combined output responsive to the first andsecond driver circuit power supplies, the shutdown circuit disabling thehigh voltage power supply if the combined output decreases below asecond threshold level.
 6. The protection circuit according to claim 4,wherein the driver circuit comprises a plurality of driver circuits,each of the driver circuits being associated with a respective source ofvideo signals and a respective cathode, and the shutdown circuitdisables the high voltage power supply if any of the first and secondpower supplies of the plurality of driver circuits decrease belowrespective first and second threshold levels.
 7. The protection circuitaccording to claim 4, wherein the shutdown circuit comprises aself-biasing latch circuit that latches to a first state when either oneof the driver circuit power supply voltages decreases below thethreshold level.
 8. The protection circuit according to claim 4, whereinthe driver circuit power supplies are disposed on a first circuit board,the driver circuit is disposed on a second circuit board, and the highvoltage power supply is disposed on a third circuit board.
 9. Theprotection circuit according to claim 1, further comprising an X-rayprotection sensor coupled to the shutdown circuit, the shutdown circuitdisabling the high voltage power supply if the output of the X-rayprotection sensor exceeds a second threshold level.
 10. The protectioncircuit according to claim 9, wherein the shutdown circuit comprises aself-biasing latch circuit that latches to a first state when the drivercircuit power supply voltage decreases below the threshold level, theX-ray protection sensor being coupled to the self-biasing latch.
 11. Thevideo display apparatus comprising: an anode and a cathode forgenerating and controlling beam currents; a source of video signals; adriver circuit coupled to the source of video signals and the cathode,the driver circuit coupled to a driver circuit power supply, the drivercircuit causing a cathode voltage to be generated on the cathode inresponse to the video signals and the driver circuit power supply; ahigh voltage power supply coupled to the anode for providing highvoltage accelerating potential in the display device; and a shutdowncircuit coupled to the driver circuit power supply and the high voltagepower supply, the shutdown circuit disabling the high voltage powersupply if the driver circuit power supply voltage decreases below athreshold level.
 12. The video display apparatus according to claim 11,wherein the driver circuit is disposed on an integrated circuit and thedriver circuit power supply provides at least +215V.
 13. The videodisplay apparatus according to claim 11, wherein the shutdown circuitcomprises a self-biasing latch circuit that latches to a first statewhen the output from the driver circuit power supply voltage decreasesbelow the threshold level.
 14. The video display apparatus according toclaim 11, wherein the driver circuit power supply comprises first andsecond driver circuit power supplies, the shutdown circuit disabling thehigh voltage power supply if either one of the first or second powersupply voltages decreases below respective first and second thresholdlevels.
 15. The video display apparatus according to claim 14, furthercomprising a combined sensing node that provides a combined outputresponsive to the first and second driver circuit power supplies, theshutdown circuit disabling the high voltage power supply if the combinedoutput decreases below a second threshold level.
 16. The video displayapparatus according to claim 14, wherein the shutdown circuit comprisesa self-biasing latch circuit that latches to a first state when eitherone of the driver circuit power supply voltages decreases below thethreshold level.
 17. The video display apparatus according to claim 11,further comprising an X-ray protection sensor coupled to the shutdowncircuit, the shutdown circuit disabling the high voltage power supply ifthe output of the X-ray protection sensor exceeds a second thresholdlevel.
 18. The video display apparatus according to claim 11, whereinthe shutdown circuit comprises a self-biasing latch circuit that latchesto a first state when the driver circuit power supply voltage decreasesbelow the threshold level, the X-ray protection sensor being coupled tothe self-biasing latch.